The general rule is to use adjusted age. So born at 29 weeks is approximately 11 weeks early, taking into account twins are usually 36-38 weeks let's call 8 weeks before they would have come full term. So if they are 9 months subtract the 8 weeks (let's call it two months) and they should be at least at the developmental level of a 7 month old, if that makes any sense at all.

Being born early should have automatically qualified them for early intervention services, which are very helpful for these kinds of things.